The mechanism behind this link appears to be a type of intestinal bacteria, the Boston research team said.

Specifically, they looked at Fusobacterium nucleatum, which is among hundreds of types of bacteria found in the large intestine. It's believed to play a role in colon cancer.

The researchers tracked the diets of more than 137,000 people for decades and examined more than 1,000 colon tumor samples.

They found that people who ate a diet high in whole grains and fiber had a lower risk of colon cancer containing F. nucleatum, but not for colon cancer without this type of bacteria.

"Though our research dealt with only one type of bacteria, it points to a much broader phenomenon -- that intestinal bacteria can act in concert with diet to reduce or increase the risk of certain types of colorectal cancer," said study co-senior author Dr. Shuji Ogino of the Dana-Farber Cancer Institute.

"Our findings offer compelling evidence of the ability of diet to influence the risk of developing certain types of colorectal cancer by affecting the bacteria within the digestive tract," Ogino added in a Dana-Farber news release.

Dr. Andrew Chan, the study's co-senior author, said these data are among the first in humans that show a connection between long-term dietary intake and the bacteria in tumor tissue.

"This supports earlier studies that show some gut bacteria can directly cause the development of cancers in animals," added Chan, who is with Massachusetts General Hospital in Boston.

"The results of this study underscore the need for additional studies that explore the complex interrelationship between what someone eats, the microorganisms in their gut, and the development of cancer," Chan said.